The major aim is to determine the mechanisms of the biosynthesis and interconversion of purine nucleotides and their derivatives in microorganisms. Auxotrophic mutants are used to unravel the essential steps, to obtain genetic alterations of the reactions, to determine the interplay of purine biosynthesis with that of vitamins such as thiamine and folate cofactors. Genetic control of the expression of the enzymes is studied by identifying multigenic units of functional control (operon) and determining the factors which control the expression of genes. Special methods for enrichment and amplification of genes have been developed for these purposes. Particular attention will be placed on the metabolic mechanisms which regulate these processess, the genetic devices which alter them, and mutational events which impair or modify their regulation. Special reactions are under consideration 1) the multienzyme system involved in the formation of the first intermediate, phosphoribosylamine, 2) the IMP-cyclohydrolase-transformylase complex, 3) IMP dehydrogenase, 4) XMP aminase, 5) GMP reductase, 6) purine phosphoribosyltransferases and 7) purine nucleoside kinases.